Be VERY careful about the way you hook it up. Generally Amp A output will go to one of the mains, Amp B output to the other main. You will find a monitor out on the board and this can be connected to your powered monitor.
The extra power amp can be used for another monitor system or . . .?
I don't think you should run Amp B output to the power amp input. It MIGHT give you a bunch of power for mono use, but it might be very dangerous. Find an experienced sound person in your area to show you the various ways you can configure your new system. Do this BEFORE you experiment.
DonM